Modern communications systems are increasing in bandwidth requirements, causing greater needs for broadband antennas. Nature, in the present physics may impose fundamental limitations on instantaneous gain bandwidth relative to antenna size and shape. The thin ½ wave wire dipole antenna can have 3 dB gain bandwidth of 13 percent and 2.0 to 1 VSWR bandwidth of only 4.5 percent. This is often not adequate. Broadband dipoles are an alternative to the wire dipole. These preferably utilize cone radiating elements which are better fitted to wave expansion, rather than thin wires. A biconical dipole having, for example, a conical flare angle of ½π radians has essentially a high pass filter response, from a lower cut off frequency. Such an antenna provides higher bandwidth, and a response of 10 or more octaves may be achieved.
In current, everyday communications devices, many different types of conical antennas, such as biconical dipoles, conical monopoles and discone antennas are used in a variety of different ways. These antennas, however, are sometimes expensive or difficult to manufacture and flat planar antennas may be preferable. Antenna shapes may be classified as linear, planar or 3 dimensional.
Many applications, such as land mobile, may require thin planar antennas with vertical polarization when mounted in a horizontal plane. Such antennas can be planar monopoles, sometimes known as microstrip “patch” antennas. The advantages of these antennas including printed circuit manufacture, being mountable in low profile, and having high gain and efficiency have made them the antennas of choice in many applications. However, microstrip patch antennas typically are efficient only in a narrow frequency band. They are poorly shaped for wave expansion, such that microstrip antenna bandwidth is proportional to antenna thickness. Bandwidth can even approach zero with vanishing thickness (for example, see Munson, page 7-8 “Antenna Engineering Handbook”, 2nd ed., H. Jasik ed.).
Simple antennas can provide quadratic “single dip” frequency responses, akin to resonant circuits. For instance, a center fed ½ wave wire dipole has an impedance response similar to a series resonant circuit plus a resistor. Multiple tuning has been described as a way to increase instantaneous gain bandwidth from small, simple antennas. In multiple tuning, an antenna may exhibit a rippled frequency response of many “dips” and “peaks”, corresponding to staggered resonances in frequency. Wheeler has shown that multiple tuned antennas can provide up to 3π the bandwidth of single tuned antennas: H. Wheeler, “The Wide-Band Matching Area for a Small Antenna”, IEEE Trans. Antennas and Propagation, Vol. AP-31, No. 2 March 1983.
External impedance compensation networks, e.g. of the inductor capacitor (LC ladder) type, may be used to increase bandwidth by multiple tuning single tuned narrowband antennas. The LC network may connect at the antenna driving points between the antenna and the feedline, and the antenna becomes the final resonant section and a load, to a cascade of resonant filter sections. It may be preferable however to obtain the multiple tuned broadband responses directly from the antenna structure, without external compensation networks, for ease of manufacture, power handling and efficiency.
Filter theory may be applied to antenna responses, and multiple tuned frequency responses tailored to polynomials. For example, a Butterworth polynomial may be used for minimal ripple or a Chebyshev polynomial for maximum bandwidth to a controlled ripple.
The bent stacked slot antenna (BSSA) achieves a relatively wide bandwidth and small size and makes use of a center strip of a middle patch as an integrated impedance matching unit. An example of such an antenna is described in the European published patent application EP 795926. However, a disadvantage with the BSSA type of antenna is the relatively narrow bandwidth.
U.S. Pat. No. 5,003,318, to Berneking et al. entitled “Dual Frequency Microstrip Patch Antenna With Capacitively Coupled Feed Pins” describes a planar ground plane antenna with two coaxial feeds or ports. Two separate antennas are collocated in space, each single tuned.
U.S. Pat. No. 6,501,427 to Lilly et al. entitled “Tunable Patch Antenna” is directed to a patch antenna including a segmented patch and reed like MEMS switches on a substrate. Segments of the structure can be switched to reconfigure the antenna, providing a broad tunable bandwidth. Instantaneous bandwidth may be unaffected however.
U.S. Pat. No. 7,126,538 to Sampo entitled “Microstrip antenna” is directed to a microstrip antenna with a dielectric member disposed on a grounded conductive plate. A patch antenna element is disposed on the dielectric member.
U.S. Pat. No. 7,109,926 to du Toit entitled “Stacked patch antenna” discloses a stacked antenna, including a lower patch which may include a coplanar microstrip capable of feeding the stacked antenna and an upper patch which may include a slot-like part thereon and coupled to the upper patch. This antenna also requires a ground plane.
There is a need for a relatively thin or horizontally planar antenna that has a wider instantaneous bandwidth, is more omnidirectional, is for vertical polarization transmission and reception and/or does not require a ground plane.